While detection of early colon cancer can improve survival significantly, currently used detection techniques are impractical for mass screening. A realistic program will be developed, designed to select patients at risk for colon cancer by serial stool occult blood testing at home. This would select a small group in which an aggressive diagnostic approach with current techniques could be applied, consisting of sigmoidoscopy, barium enema with air contrast and colonoscopy. The relative value of these procedures will be assessed including directed biopsy through the sigmoidoscope and directed biopsy and cytology through the colonoscope. Newer detection techniques will be developed and evaluated based, on pulsatile rectocolonic lavage through the sigmoidoscope and colonoscope. Washings obtained by lavage will be studied by cytologic, enzyme, immunologic and isotopic labelling techniques to provide useful information in patients at risk for colon cancer and to suggest possible new approaches to early detection. The primary objective is for the detection of colon cancer in asymptomatic patients. A sub-study will evaluate this approach in patients with antecedent primary cancer and polyps of the colon. For each of the two groups, randomization into study and control groups will be done. A questionnaire will be used to identify asymptomatic and pre-symptomatic patients and the rate of detection of cancers and polyps will be compared in the asymptomatic and pre-symptomatic groups. Pathologic staging of the cancer at the time of detection will be the primary basis for comparison of study and control groups. Survival will require a longer period of follow-up.